Patterns of wetland plant diversity along estuarine stress gradients of the Elbe (Germany) and Connecticut (USA) Rivers

Abstract

Background: Estuaries are characterised by salinity gradients and regular flooding events. These environmental factors form stress gradients, along which species composition changes.

Aims: Analyse and compare patterns of plant species diversity along the estuarine salinity and flooding gradients of the Elbe and Connecticut Rivers.

Methods: Vegetation was sampled at three elevations (low, mid, high) in five sites of each marsh type (fresh, brackish, salt) in both estuaries. Patterns of species density (SD) and evenness (E) along the gradients were analysed and compared between the two estuaries with three-factor ANOVAs.

Results: The regional species pool was 33% higher for the Connecticut than for the Elbe. SD of fresh marshes (19 ± 2.2) was more than twice in the Connecticut than in the Elbe. We found an overall increase in SD from low to high elevation and from salt to freshwater marshes in both estuaries. However, SD and E were strongly depressed at intermediate elevations in the Elbe fresh and brackish marshes.

Conclusions: Although diversity patterns in the two estuaries show overall similarities, patterns of SD and E differ, when particular elevational zones and marsh types are compared. We hypothesise this to be due to evolutionary and historical influences on the regional species pools, shaping the impact of local biotic and abiotic processes.

Keywords:

• Connecticut estuary
• Elbe estuary
• environmental stress gradient
• estuarine marshes
• flooding gradient
• intercontinental study
• plant species diversity
• salinity gradient
• vegetation zonation

Acknowledgements

We would like to thank Andrew Baldwin and C. John Burk for stimulating discussions on the project and helpful comments on various drafts of the manuscript and the anonymous referees for their constructive feedback. We further thank Franziska Rupprecht, Paul Wetzel, Janne Jensen and John Burk for their assistance in the field. Finally, we thank the University of Hamburg for providing the Ph.D. grant to Gesina Engels and Smith College and the University of Hamburg for a faculty exchange grant for Kai Jensen.